The present invention is related to circuits in which a field-effect transistor device controls power transfer from an alternating polarity electrical power supply to a load means, particularly when such field-effect transistor devices are capable of being integrated in monolithic integrated circuits.
Various solid state devices have been used in circuits as the primary means for controlling power transfer from an alternating polarity electrical power supply to whatever kind of load means is of interest for use in the circuit. For instance, planar bipolar power transistors have been used but these are devices which are not bidirectional by nature and which exhibit an inherent, more or less irreducible, minimum power dissipation characteristic even when fully switched on. And to be switched fully on, bipolar power transistors require a substantial amount of base current, i.e., control current, especially for higher collector, or load, currents. Furthermore, they are also subject by nature to thermal runaway.
Perhaps more commonly used for controlling alternating polarity power supplies are thyristors of various kinds such as silicon controlled rectifiers and triacs. Such thyristors are switching devices primarily used in alternating polarity power supply control circuits because of their capability for handling relatively large power dissipations when switched fully on, and for withstanding substantially reversed voltages when switched fully off. An advantage of these devices over bipolar power transistors is that they require little electrical power at device control gates whether operating in the off condition or in the on condition.
However, such thyristors also have several disadvantages such as being a latching switch, that is, operating only in fully on or fully off states. Further, thyristor devices can be switched off by sufficiently reducing the current therethrough, and can be switched on by sharp voltage transients thereacross--both results being obtained without any action taking place at the control terminal of the thyristor device. Hence, the control terminal of the thyristor has relatively little continuous control capability. This same control terminal, in many situations, cannot be electrically isolated simply and inexpensively from the load circuit and may require a large triggering current to switch on the thyristor device. Finally, a thyristor device cannot be easily provided in a monolithic integrated circuit with other circuit components because of its structure and power dissipation.
Hence, better primary power controlling devices are desired for use in controlling power transfer from alternating polarity electrical power supplies and alternating polarity operated circuits. Particularly useful would be a device which could be easily provided in a monolithic integrated circuit along with other circuit components, at least some of which would also be used in controlling power transfer from the alternating polarity power supply used. This will require that such a device not have too large a resistance if switched fully on, despite substantial current loads, but which would have a structure easily fabricated in such an integrated circuit. Further, the device should have a bidirectional current conduction capability for circuits in which current rectification is not desired.
Field-effect transistor devices can have many of the characteristics just described, including having a very symmetrical bidirectional current conducting capability when on. This is certainly so for metal-oxide-semiconductor field-effect transistor (MOSFET) devices which have the advantage of having the gates therein very well isolated from the channel regions of the device. This isolation aids in providing a circuit to operate the field-effect transistor device when both the circuit and these devices are formed in a monolithic integrated circuit chip, a difficult arrangement when the integrated circuit is to operate with an alternating polarity power supply. Such circuits must permit the operation of other circuit component devices in the monolithic integrated circuit while also controlling power transfers from the alternating polarity power supply through operating the primary power transfer control field-effect device.
Electronic component device theory shows that field-effect transistors are operated by controlling the voltage appearing between the gate thereof and the connection to that one of the two channel regions therein which is effectively serving as the transistor source. Difficulties arise in those circuits using a field-effect transistor to control power transfers from an alternating polarity power supply because the two connections to the channel region of such a transistor serve alternately as the source rather than one of them serving continually as the source.
Certain field-effect transistor structures are especially useful for providing a field-effect transistor capable of controlling power transfer from an alternating polarity power supply to a load means. Such transistors should have low channel resistance when operated fully on--on the order of tenths of ohms--if they are to be successfully used in a monolithic integrated circuit. Then these transistors, when passing several amps of current, will not cause the circuit to suffer heat dissipation sufficient to disrupt the operation of other circuit components. Further, this channel resistance in a fully turned on device should be more or less symmetrical so there are no current rectification effects occurring. And, of course, the transistor device structure should be capable, when switched off, of withstanding, without breakdown, voltages at least as large as the peak voltage provided by the alternating polarity supply used. In this regard, the applications referenced above entitled "Semiconductor Apparatus" teach various devices, effectively field-effect transistors, which exhibit one or more of these desired characteristics.
The application referenced above entitled "Alternating Polarity Power Supply Control Apparatus" by Hendrickson shows several circuits making use of such a field-effect transistor to control power transfer from an alternating polarity electrical power supply to a load means. One of these circuits is shown in FIG. 1 of the present application. This circuit operates using an enhancement mode, p-channel, MOSFET, 10, for controlling power transfers from an alternating polarity electrical power supply, 11, to a load means, 12, or alternatively, to a selected one of three other load means, 30, 31, or 32 provided at other locations in the circuit (these alternative loads are shown by dashed lines). As the above-referenced Hendrickson application indicates, there are several other essentially equivalent circuit versions of the circuit of FIG. 1 indicated therein using other kinds of electronic components, as well as some entirely different circuits.
An advantage of the circuit shown in FIG. 1 herein is that the circuitry for controlling power transfers through transistor 10 from supply 11 to load means 12 can be operated from electrical power supplied solely by alternating power supply 11. That is, a control switch means, 33, is shown for operating transistor 10 where control switch means 33 can be operated solely from voltage developed across a capacitor, 27 derived ultimately from supply 11.
Of particular note in FIG. 1 of the present application is the explicit showing therein of the effecitve, but parasitic, circuit components inherent in transistor 10 which are presented in equivalent "lumped" form, all of these being present as a result of the actual physical structure of transistor 10. Of course, every transistor physical structure leads to having, effectively, parasitic circuit components associated therewith. However, such parasitic components are more likely to be significant in value for a power control transistor, such as transistor 10, compared to a signal control transistor because the power transistor is usually of a relatively large physical size when compared to transistors used for controlling signals only. Thus, the parasitic components are explicitly shown with transistor 10 only in FIG. 1 even though such parasitic components are also associated with the structures of the other transistors shown in FIG. 1. The assumption is that these other transistors have associated parasitics that will have a relatively insignificant effect on circuit operation.
Field-effect transistor 10, being a p-channel MOSFET, is provided in a substrate material of n-type conductivity. The channel connection regions, 15 and 16, which terminate the ends of the channel region in transistor 10 and can serve as source and drain regions therein, are formed by diffusion or implantation of p-type conductivity impurities into the substrate material. Parasitic diodes are formed in the structure of transistor 10 by the semiconductor pn junctions occurring between regions 15 and 16, on the one hand, and the substrate of transistor 10 on the other. These diodes are designated 17 and 18 in FIG. 1.
Also associated with these pn junctions are parasitic capacitances, 19 and 20, and parasitic resistances 21 and 22. Further parasitic capacitances present are a channel-to-substrate capacitance, 23, and a gate-to-channel capacitance, 24. Two other parasitic capacitances, 25 and 26, are shown which are each effective between gate 14 and one of the channel terminating regions 15 or 16. All of these parasitic components will have more or less of an effect on the operating behavior of transistor 10, and so in the behavior of the circuit in which transistor 10 is provided. The significance of these effects depends on the conditions existing in such a circuit. Of course, capacitance 24 is essential for switching on transistor 10 by forming a channel, yet this capacitance and the other parasitic components shown with transistor 10 are normally desired to contribute as insignificantly as possible to the circuit operation.
At sufficiently low frequencies, the parasitic capacitances shown in connection with transistor 10 in FIG. 1 will not be significant factors in the operation of the circuit of this figure. Also, the leakage resistances 21 and 22 of FIG. 1 are usually sufficiently large so that they will not be significant in the operation of this circuit.
Further, note that load means 12 could also have a reactance component thereto but has been shown and will be described as being resistive for ease of understanding and exposition. This is also true of the alternative to load means 12, that is load means 30, 31, and 32. The alternative uses of these load means in the circuit of FIG. 1 are described in the above-referenced Hendrickson application entitled "Alternating Power Supply Control Apparatus".
The two enhancement mode, p-channel, metal-oxide-semiconductor field-effect transistors (MOSFET'S), 28 and 29, connected across alternating polarity power supply 11 are connected to operate as diodes. In operating in this manner, transistor 28 appears to be a diode having its cathode connected to alternating polarity power supply 11 and an anode connected to an energy storage capacitor, 27. The same description fits transistor 29. As indicated in the above-referenced Hendrickson application entitled "Alternating Power Supply Control Apparatus", the primary power transfer control transistor 10 and the signal controlled enhancement mode, p-channel MOSFETS, 34 and 35, along with transistors 28 and 29 (depending on which of loads 12, 30, 31 and 32 are actually used) can each have its substrate connection electrically connected in common with each of the other transistors as would occur if they were jointly formed in a single monolithic integrated circuit chip. This is not necessarily true for the substrate connection for transistors 28 and 29 for certain choices in selecting one of loads, 12, 30, 31 and 32.
The sole source of power used to operate the circuit of FIG. 1 is alternating polarity power supply 11. Supply 11 not only provides power for controlled transfer to load means 12 (the load chosen for purposes of the following description of FIG. 1), upon being selected to do so by appropriately activating switch means 33, but also provides power to be stored in capacitor 27 to operate circuitry of switching means 33 and perhaps other circuits. Of course a separate power supply means could be used in place of capacitance 27, and this must be done to use a depletion mode device in place of the enhancement mode transistor 10 as described in the Hendrickson application referenced above entitled "Alternating Polarity Power Supply Control Apparatus". In the arrangement of FIG. 1, with constant polarity voltage being supplied to switch means 33 from across capacitor 27, transistors 34 and 35 and the associated switch control circuitry, 36, are all electrically energized by the stored electrical energy provided in capacitor 27. In operation, switch means 33 has either transistor 34 on and transistor 35 off, or vice versa, as determined by switch control circuitry 36, and so these transistors together operate in series as a single pole, double throw switch.
In the situation where transistor 34 is switched on while transistor 35 is switched off--thereby effectively shorting the gate region, 14, of transistor 10 to the substrate connection, 13, of transistor 10--supply means 11 will charge capacitance 27. When the side of supply 11 not connected to load means 12 is positive, the charging current will flow through channel terminating region 16, parasitic diode 18, capacitance 27 and transistor 28 serving as a diode thereby charging capacitance 27. When supply 11 changes polarity, a charging current will flow through terminating region 15 of transistor 10, parasitic diode 17, capacitance 27 and transistor 29 serving as a diode.
Note further, there will be little discharging of capacitor 27 as supply 11 output voltage polarity changes (depending on how switching control circuitry 33 is implemented). This is because of the reverse biased nature of all of the diodes, including the parasitic ones, between the positive side of capacitance 27 and the negative side thereof as shown in FIG. 1.
In this situation, transistor 10 will be held fully off because of the effective short occurring between gate region 14 and substrate connection 13 through transistor 34. Gate 14 will follow the substrate which will also be within a voltage drop across one of the parasitic diodes 17 or 18 of the positive side of supply 11. Thus, the threshold voltage of transistor 10 will never be exceeded by the voltage occurring between gate region 14 and whichever the terminating region 15 or 16 is positive with respect to the other. Device theory indicates in these circumstances that the transistor 10 will be off.
However, if the control situation is reversed and transistor 35 is switched fully on with transistor 34 being switched off to thereby effectively short the negative side of capacitor 27 to gate region 14, transistor 10 will be switched fully on. This occurs because in these circumstances gate region 14 is held negative with respect to the substrate by the voltage appearing across capacitor 27. Yet, the substrate is still always within a diode voltage drop of the positive voltage value appearing on one side of transistor 10 or the other, i.e., one of terminating regions 15 or 16, through parasitic diodes 17 and 18 (excluding any circuit transients in this consideration). As a result, the gate of transistor 10 is held negative with respect to whichever of channel terminating regions 15 and 16 is positive, that region being, device theory indicates, the channel terminating region then serving as the transistor 10 source. Thus, for sufficient voltage across capacitor 27, device theory indicates that transistor 10 will be on.
The preceding circuit operation description made no mention of the parasitic capacitances associated with transistor 10 on the assumption of sufficiently low frequency operation of supply 11.
However, for sufficiently high frequencies of polarity alternation in the output voltage of alternating polarity power supply means 11, the operation just described will no longer be accurate. This is primarily because of the presence of these parasitic capacitances associated with transistor 10 as shown in FIG. 1.
At least three detrimental circuit operation effects of possible significance can occur because of the presence of these parasitic capacitances. First, the charging of parasitic capacitances 19 and 20, and capacitance 27, with transistor 10 off can lead to bipolar transistor action between terminating regions 15 and 16 in the form of an effective pnp transistor which would tend to provide a more or less conductive pathway between terminating regions 15 and 16 which are intended to be electrically isolated from one another in these circumstances. Second, the charge on these parasitic capacitances may lead to delays in the intended operation of transistor 10 because of the charge in the parasitic capacitors tending to maintain earlier existing conditions about transistor 10 until these parasitic capacitors have been discharged. This can lead to transistor 10 responding slowly, incompletely or not at all to the electrical signals supplied intended to control this transistor.
Finally, the charging of the parasitic capacitors leads to a voltage thereacross which can add to the voltage being provided by alternating power supply 11 as it changes polarity. This situation can either cause transistor 10 to breakdown or will require the breakdown voltages associated with transistor 10 to be approximately twice as large as the peak voltage being supplied by alternating polarity power supply 11.
These undesirable effects are likely to be encountered with the use of a large physical size transistor as is usually necessary for controlling transfer of substantial amounts of power from alternating polarity supply 11 to a load means in many kinds of power transfer control circuits. Thus, means for eliminating these parasitic effects are desirable features in circuits having field-effect transistor devices used for controlling substantial transfers of power to load means from alternating polarity power supplies having sufficiently high frequencies of polarity alternation. A further desirable feature would be accomplishing this in a circuit which permits providing constant polarity power to other circuit components, including auxiliary control components used in controlling the primary transfer control field-effect transistor device, and yet requires only the presence of the alternating polarity power supply as the single electrical power source.